The present invention relates to a semiconductor device in which a semiconductor chip is sealingly encapsulated in resin, and particularly to such a device in which the resin encapsulates the semiconductor chip with a space therearound. The invention also relates to a method for fabricating such a device.
For packaging semiconductor chips, both resins and ceramic materials have commonly been used. The use of resin is preferred, however, for reasons of economy.
FIG. 1 shows a conventional semiconductor device in which a semiconductor chip 1, bonding wires 2, and a die pad 3 are directly covered with resin 4. Such a semiconductor device is fabricated by soldering the semiconductor chip 1 to the die pad 3, connecting lead wires 5 to electrodes of the semiconductor chip 1 via the bonding wires 2, disposing this assembly in a metal mold, and then encapsulating the assembly with resin 4.
Such a structure of a semiconductor device does not require any step of assembling solid members in sealing the semiconductor chip, contrarily to that using ceramic packaging, and thus it is suitable for mass production with the merit of low material costs.
On the other hand, however, since the semiconductor chip 1 is directly in contact with the resin 4, and the thermal expansion coefficient of the resin 4 is different from that of the semiconductor chip, stress is produced upon temperature variation, and as a result the contacts between the bonding wires 2 and the semiconductor chip may be degraded and/or cracks in the latter may occur, causing a malfunction of the semiconductor device. In view of this fact, having a member made of resin in direct contact with the chip is not recommendable for a semiconductor chip 1 which generates relatively large amounts of heat.
A semiconductor device including a package member of resin having an opening in one side thereof in which a semiconductor chip is inserted and a cover member of the same resin material for covering the opening and adapted to be bonded thereto, whereby the semiconductor chip is separated from the resin members, has been proposed in "Practical IC Techiques", page 137, FIG. 6.3(e), Nippon Microelectronics Association, Jan. 25, 1981.
The assembling procedure of the proposed semiconductor device is similar to that of the conventional device using ceramic members and, therefore, its productivity is as low as that of the device using ceramic members. Further, if the chip to be encapsulated by the resin package is a read-only memory erasable with ultraviolet rays, the cover member must be transparent to ultraviolet rays. However, no presently known resin material provides both satisfactory sealing and transparency to ultraviolet rays.
In order to overcome this problem, it may be considered to prepare the cover member using a material different from that of the package, such as quartz glass. However, since the thermal expansion coefficient of resin is considerably different from that of quartz glass, the seal between the package member and the cover member may be broken due to temperature variations during the assembling procedure, ambient temperature variations, and/or heat generation of the semiconductor chip in normal operation, and, in some cases, there may be cracks produced in either or both members. Therefore, it is usual to employ a ceramic package member having a thermal expansion coefficient substantially the same as that of the cover member of quartz glass. Thus, for the reason explained above, material costs are high and productivity low.